In volatile substance diffuser devices with wick, there is a need to allow the user to easily detect end-of-life of the volatile substance and therefore perceive the need to replace the container of the substance with a new one. These devices are generally plugged into out-of-sight household power outlets, due to which the level of liquid remaining in the container is not easily seen at first sight.
There are several solutions for converting end-of-life into an electric indicator that can then imply some type of alert to the user. These systems are based on the measurement of a change in physical properties, such as electrical conductivity, dielectric capacity, weight, temperature or light interaction.
With regard to the light interaction-based liquid detection systems, we can differentiate between detection systems for detecting liquid at the bottom of the container or inside the wick.
Detection systems for detecting liquid at the bottom of the container, for example, by means of light transmission, have the following drawbacks:                High degree of dispersion due to the lack of geometric accuracy of the bottles generally used.        Influence of external conditions (sunlight, reflections, . . . ).        End-of-life is detected when there is still liquid inside the wick and due to which evaporation of the formulation still occurs.        
On the other hand, there are systems based on light transmission or refraction through the upper part of the wick. However, these systems require the wick to be transparent to light or that, in the presence of liquid inside the wick, a sufficient amount of light can be refracted through the cavities of the wick and reach the sensor.
These systems have the following drawbacks:                Need to adjust the position of the light emitter/sensor based on the refractive index of the fragrance.        Influence of external conditions (sunlight, reflections, . . . ).        
U.S. Pat. No. 7,164,849 describes a device wherein the vaporisable liquid includes an IR-absorbing material. When there is sufficient vaporisable material in the container, the infrared light emitted cannot be reflected by the material towards the IR sensor. The infrared light is absorbed by the vaporisable material in such a manner that the IR sensor does not detect the IR light. When the IR sensor detects IR light the alarm is activated.
In North American patent application US 2006019962 A1, the light emitter and receptor may be disposed in any position of the diffuser around the wick and provided that the emitter and receptor are aligned facing each other. When the container is full or contains a certain amount of liquid, the wick absorbs the liquid. In this case, when there is liquid remaining in the container and the wick is wet, the light emitted by the emitter is refracted through the saturated porous wick, allowing the light to be detected by the sensor.
The refractive index of the liquid and the wick material determine the amount of light emitted by the emitter that is detected by the receptor. For example, as the refractive index of water is very low, the light emitted by the emitter is not refracted through the wick. Therefore, the refractive index of the liquid must be sufficiently high so as to refract the light through the wick and ensure that the light will be detected by the sensor.
In an alternative option, the emitter and light detector may be disposed on the container. One is disposed on the upper part and the other on the lower part of the container. When the container is empty, the light may pass through it. However, when there is liquid the light is refracted or reflected and does not reach the sensor.
In another option, the emitter and receptor are disposed around the container but not aligned. When there is liquid the light is scattered through the container and reaches the receptor, but when there is no liquid the light travels in a straight line and does not reach the receptor. It detects when the container is completely empty or almost empty, depending on the position of the sensor.
The drawbacks of these systems are the following:                Light transmission through the wick is not very efficient and requires a high-sensitivity sensor.        In order for light transmission to be sufficiently efficient, specific wick characteristics are required (pore size, pore percentage, . . . ).        Possible interference by ambient light.        The light emitter and sensor must be aligned facing each other, which entails technical difficulties for reducing position tolerances.        
On the other hand, international application WO2007/138247 describes an end-of-life detection system based on two methods:                Use of light refraction through the glass container.        Use of total internal reflection (combination of refraction and reflection).        
The light source is adapted to direct IR or visible light towards the container at an angle that is substantially within or between:                a) a critical angle of incidence for an interphase between the liquid and the container; and        b) a critical angle of incidence due to an interphase between the air and the container.        
The critical angle is preferably a critical angle for total internal reflection. The sensor is disposed in such a manner as to receive light from the emitting source that has entered the container and has been reflected in the interphase between the container and the air inside the container. Or, to receive light from the light source that has entered the container and has been refracted in the interphase between the container and the air. The sensor must be disposed in relation to the light source in such a manner that the liquid of the container is present at detection level. The emitter and detector must be in the line of sight.
The drawbacks of this system are the following:                It requires an adequate positioning of the emitter so that the light enters the container at certain angles.        The container must be made of glass. The emitter/receptor system depends on the thickness and purity of the glass.        The container and emitter/receptor system must be designed in such a manner that the wick does not interfere with the light path.        In the total internal reflection method there will always be fragrance remaining in the container and an additional end-of-life indication system is required (timer, . . . ).        